In recent years there has been a tremendous worldwide effort to develop motor vehicles operated by a fuel other than a petroleum distillate. Such efforts have resulted in hydrogen powered vehicles, steam powered vehicles and electric powered vehicles, the latter of which have been adopted as the most promising alternative to the normal gasoline consuming internal combustion engine. With the adoption of the electric motor vehicle as the proposed answer to such a crucial problem, there has been a need to develop an on-board source of electrical power. Since fuel cells, nuclear generators and other systems have not been perfected to a degree which allows their immediate use on motor vehicles, most developmental efforts in commercializing electric powered motor vehicles now involve the use of storage batteries carried by the vehicle. For that reason, various types of storage batteries have been developed with the objective of increasing the range between charging, without sacrificing the operating speed and/or decreasing the time and effort required in actually charging the on-board storage battery. This objective has now been generally satisfied by zinc-chloride battery of the type commercialized by Energy Development Associates and generally disclosed in U.S. Pat. No. 3,935,024. This patent is incorporated by reference herein as background information regarding the power supply or main storage battery to which the present invention is particularly applicable. When employing a zinc-chloride battery, a plurality of cells are combined to create a source of direct current voltage having a voltage level sufficient to drive efficiently the motor used in propelling the vehicle. In practice, the zinc-chloride batter has a normal operating voltage of 120 volts D.C. This voltage is applied to the storage battery during a charging operation and it is created by the battery during normal discharge or driving of the vehicle. Consequently, the basic on-board power supply of a motor vehicle of this type has an available main voltage level, which in practice is 120 volts D.C. This allows efficient operation of a drive motor. Zinc-chloride and other metal halogen storage batteries require at least one motor driven pump. This pump circulates electrolyte through the cells during both charging and discharging of the battery. Generally, this type of battery incorporates a second pump usually a gas pump which is used to exhaust halogen, in practice chlorine, from the main battery compartment to a storage compartment wherein the chlorine is formed into a chlorine-hydrate. This action occurs during charging. In addition, this second pump may be used during discharging to facilitate the flow of chlorine from the hydrate storage area to the main battery compartment. Since these two pumps are to be used both in a discharging mode and a charging mode, they are driven by D.C. motors which normally respond to a voltage level corresponding to the available voltage on the vehicle. Generally this level is the high voltage level of the main storage battery, in practice 120 volts D.C. By providing one or more D.C. motor driven pumps in a battery system for use on a vehicle, it is necessary to provide a control system for these pumps so that they are operated in accordance with the desired parameters existing at any given time in the battery system. Thus, the pumps must have variable speed which is accurately controlled to optimize the chemical and physical parameters employed in the complex circulatory and chemical systems embodied in a zinc-chloride type of battery. Controlling the pumps is somewhat complex procedure if the pumps are to be a factor in optimizing the operation of the battery. Such optimization is required to obtain uniform operation and long life of a battery for powering a motor vehicle. Control of these pumps is further complicated by the exigencies of the variable conditions under which the main battery must operate. When the pumps are stopped, the cells do not produce maximum voltage; however, the operation of a pump, particularly at this time, is crucial to battery start-up. During such start-up, it is essential that the battery be operated in accordance with the preselected plan to bring the battery into its starting mode. This must be done rapidly since public acceptance of the vehicle demands that the vehicle be ready to move with a minimum delay after the operator starts the vehicle. Thus, the pumps must be operable at a controlled speed at variable voltage levels of the main battery and during periods of time when the main battery has no output voltage, i.e. when the only available on-board electricity comes from an auxiliary storage battery of the 12 volts type. In addition, since one critical factor of commercial acceptance in a vehicle is the mileage between charges, the pumps must not be a major consumer of available stored energy. This requires accurate monitoring of adverse conditions when the pump or pumps draw substantial current. Thus, it is necessary that the pumps not be allowed to draw substantial currents for prolonged times without some type of external attention.